Archaean crustal accretion at the northern border of the Congo Craton (South Cameroon). The charnockite-TTG link

Abstract

Abstract Magmatic charnockitic rocks and TTG (tonalite-trondhjemite-granodiorite) plutons intruded successively the Archaean greenstone belts of the Ntem Complex, at the northwestern margin of the Congo Craton. Geochemical data, zircon Pb-Pb ages, and Sr-Nd isotope studies, constrain the magmatic features, the genetical timing, and the geodynamic settings of these different suites. Charnockites and TTGs are characterized by high Al2O3 contents, high Na/K ratio, low Th content, LREE enrichment, HREE depletion, negative anomalies in Nb, Ta, and Ti, and positive anomalies in Sr. The Pb-Pb zircon ages indicate that charnockites were emplaced at ca. 2900 Ma and TTGs, which cross-cut the charnockites, at ca. 2830 Ma. TDM Nd mean crustal residence ages of both suites range between 3.10 and 2.93 Ga. The charnockites show slightly positive initial εNd2.9Ga (+0.3 to +1.3), whereas the TTGs have slightly negative values (+0.1 to −1.5). The charnockitic and TTG magmas may have resulted from different partial melting processes of the primitive Archaean basaltic crust and contaminated mantle, possibly in a hot slab subduction convergent regime. They contribute to a two-stage crustal growth of the Archaean craton.